Production of xylenes from syngas

ABSTRACT

This disclosure relates to the production of xylenes from syngas, in which the syngas is converted to an aromatic product by reaction with an isosynthesis catalyst and an aromatization catalyst. The isosynthesis catalyst and aromatization catalyst may be different catalysts or combined into a single catalyst. The aromatic product is then subjected to one of more of (i) xylene isomerization, (ii) transalkylation with at least one C 9 + aromatic hydrocarbon, and (iii) alkylation with methanol and/or carbon monoxide and hydrogen to increase its p-xylene content.

PRIORITY CLAIM

This application claims priority to and the benefit of U.S. ProvisionalApplication No. 62/028,486, filed Jul. 24, 2014 which is incorporated byreference in its entirety.

FIELD

This disclosure relates to the production of xylenes from syngas.

BACKGROUND

The isomers of xylene find wide and varied application. For example,meta-xylene (m-xylene) is used in the manufacture of dyes, andortho-xylene (o-xylene) is used as a feedstock for producing phthalicanhydride, which finds use in the manufacture of plasticizers. However,currently the most valuable of the xylene isomers is para-xylene(p-xylene), since p-xylene is a feedstock for terephthalic acid, whichin turn is used in the manufacture of polyester fibers and films.

The majority of p-xylene produced today is derived from crude oil viareforming of the naphtha portion of the crude into a mixture of benzene,toluene, and xylenes (BTX) and heavier aromatics. These aromatics thenundergo a variety of reactions, such as transalkylation,disproportionation, and xylene isomerization, to increase theconcentration of p-xylene. The current commercial process also requiresextraction of aromatics from non-aromatics and separation of p-xylenefrom a mixture of xylene isomers via crystallization or molecular sieveadsorption. The overall process is therefore complex. Moreover, as crudeoil prices rise, so does the feed stock price for p-xylene productionvia the current commercial routes. Recently, as crude prices have risen,the prices of coal and natural gas having fallen making syngas derivedfrom these sources cheaper on a carbon or energy equivalent basis and apotentially attractive feed for the production of basic chemicals, suchas p-xylene.

The conversion of syngas to olefins and paraffins has been widelypracticed for many years via the Fischer-Tropsch process and indirectlyvia the methanol to olefins (MTO) process. However, both of these syngasconversion routes produce only small amounts of aromatics and there istherefore a need for an improved route for converting syngas toaromatics, and particularly, p-xylene.

In a paper entitled “Direct Conversion of Syngas into Aromatics overBifunctional Fe/MnO—ZnZSM-5 Catalyst”, Chinese Journal of Catalysis,Volume 23, No. 4 July, 2002, Wang Desheng et al. report that syngas canbe converted into aromatics at high yield using a bifunctional catalystcomprising Fe/MnO mixed with Zn-ZSM-5 containing up to 7 wt % Zn underconditions including a temperature of 517° F. (270° C.), a pressure of160 psig, and a hydrogen to carbon monoxide molar ratio of 2:1. However,the aromatic product slate obtained in the process of Wang et al. iscomposed mainly of benzene and toluene, rather than the more desirablep-xylene. Further, the preferred operating conditions of theFischer-Tropsch process and aromatization process differ, limiting theconversion and selectivity to aromatics.

Another method for the conversion of syngas to higher hydrocarbons isisosynthesis, in which syngas is converted to C₄ hydrocarbons. Becausethe isosynthesis reaction occurs in the same preferred temperature rangeas the aromatization reaction, there is a desire to combine isosynthesiswith aromatization to provide a process of converting syngas toaromatics in which the yield of xylene isomers, and in particularp-xylene, is improved.

SUMMARY

The present invention is directed to the conversion of syngas over acombination of an isosynthesis catalyst and an aromatization catalyst toeffectively and cost efficiently produce a BTX-containing mixture whichcan then be subjected to isomerization, transalkylation and/ormethylation to increase the para-xylene content of the mixture. A firstfeed comprising hydrogen and carbon monoxide in a molar ratio ofhydrogen to carbon monoxide from about 0.5 to 6 is contacted with (i) afirst catalyst comprising at least one metal or compound containing ametal selected from the group consisting of Ce, Zn, Zr, and Th, and (ii)a second catalyst, which may be the same as or different than the firstcatalyst, comprising at least one medium pore size molecular sieve underconditions including a temperature of at least 350° C. and a pressure ofat least 1500 kPa (absolute) effective to produce a reaction effluentcontaining benzene, toluene, and xylenes. In some embodiments, xylenes,especially p-xylene, are recovered directly from the reaction effluent.In other embodiments, at least part of the reaction effluent issubsequently subjected to at least one of (a) contacting with a xyleneisomerization catalyst, (b) transalkylation with at least one C₉+aromatic hydrocarbon, and (c) alkylation with methanol and/or carbonmonoxide and hydrogen to increase the xylene concentration of theeffluent.

The invention further provides a catalyst system comprising a firstcatalyst comprising at least one metal or compound containing a metalselected from the group consisting of Ce, Zn, Zr, and Th, and a secondcatalyst, which may be the same as or different than the first catalyst,comprising at least one molecular sieve and at least one metal fromGroups 10-14 of the Periodic Table or compound thereof, with the firstand second catalysts located within the same reactor bed.

DETAILED DESCRIPTION

The present disclosure relates to a process for the production ofxylenes, and particularly p-xylene, from syngas. The initial syngasconversion proceeds via an isosynthesis catalyst and an aromatizationcatalyst, which may be different catalysts or combined into a singlecatalyst, to produce a hydrocarbon mixture containing benzene, toluene,and xylenes (BTX). P-xylene can then be recovered directly from the BTXproduct and/or at least part of the BTX can be subjected to alkylation,transalkylation and/or isomerization processes to enhance the p-xyleneconcentration of the product.

Definitions

For the purpose of this specification and appended claims, the followingterms are defined. The term “C_(n)” hydrocarbon wherein n is a positiveinteger, e.g., 1, 2, 3, 4, or 5, means a hydrocarbon having n number ofcarbon atom(s) per molecule. The term “C_(n+)” hydrocarbon wherein n isa positive integer, e.g., 1, 2, 3, 4, or 5, means hydrocarbon having atleast n number of carbon atom(s) per molecule. The term “C_(n−)”hydrocarbon wherein n is a positive integer, e.g., 1, 2, 3, 4, or 5,means hydrocarbon having no more than n number of carbon atom(s) permolecule. The term “aromatics” means hydrocarbon molecules containing atleast one aromatic core. The term “hydrocarbon” encompasses mixtures ofhydrocarbon, including those having different values of n. The term“syngas” means a gaseous mixture comprising hydrogen, carbon monoxide,and optionally some carbon dioxide.

As used herein, the numbering scheme for the groups of the PeriodicTable of the Elements is as disclosed in Chemical and Engineering News,63(5), 27 (1985).

Syngas-Containing Feed

The syngas-containing feed employed in the present process compriseshydrogen and at least 4 mol. %, for example at least 10 mol. %, ofcarbon monoxide, such that the molar ratio of hydrogen to carbonmonoxide in the feed is from about 0.5 to 20, preferably from about 0.5to 10, more preferably from about 0.6 to 6 or from about 0.8 to 4, andmost preferably from about 1 to 3. In a preferred embodiment, the molarratio of hydrogen to carbon monoxide is from about 0.5 to 6. The syngasmay also contain carbon dioxide such that the feed can have a H₂:(CO+CO₂) molar ratio of from 2 to 60.

The syngas can be produced from methane and/or other carbon-containingsource material. The type of carbon-containing source material used isnot critical. The source material can comprise, e.g., methane and otherlower (C₄—) alkanes, such as contained in a natural gas stream, orheavier hydrocarbonaceous materials, such as coal and biomass.Desirably, the source material comprises ≧10 vol. %, such as ≧50 vol. %,based on the volume of the source material, of at least one hydrocarbon,especially methane.

The source material can be converted to syngas by any convenient method,including those well-established in the art. Suitable methods includethose described in U.S. Patent Application Publication Nos. 2007/0259972A1, 2008/0033218 A1, and 2005/0107481 A1, each of which is incorporatedby reference herein in its entirety.

For example, natural gas can be converted to syngas by steam reforming.This normally involves the initial removal of inert components in thenatural gas, such as nitrogen, argon, and carbon dioxide. Natural gasliquids can also be recovered and directed to other processing ortransport. The purified natural gas is then contacted with steam in thepresence of a catalyst, such as one or more metals or compounds thereofselected from Groups 7 to 10 of the Periodic Table of the Elementssupported on an attrition resistant refractory support, such as alumina.The contacting is normally conducted at high temperature, such as in therange of from 800° C. to 1100° C., and pressures ≦5000 kPa. Under theseconditions, methane converts to carbon monoxide and hydrogen accordingto reactions such as:CH₄+H₂O=CO+3H₂.

Steam reforming is energy intensive in that the process consumes over200 kJ/mole of methane consumed. A second method of producing syngasfrom methane is partial oxidation, in which the methane is burned in anoxygen-lean environment. The methane is partially-oxidized to carbonmonoxide (reaction (i)), with a portion of the carbon monoxide beingexposed to steam reforming conditions (reaction (ii)) to producemolecular hydrogen and carbon dioxide, according to the followingrepresentative reactions:CH₄+ 3/2O₂=CO+2H₂O  (i),CO+H₂O=CO₂+H₂  (ii).

Partial oxidation is exothermic and yields a significant amount of heat.Because steam reforming reaction is endothermic and partial oxidation isexothermic, these two processes are often performed together forefficient energy usage. Combining the steam reforming and partialoxidation yields a third process for generating syngas from natural gasin which the heat generated by partial oxidation is used to drive steamreforming to yield syngas.

In certain embodiments, especially those employing an isosynthesiscatalyst for the initial syngas conversion, the syngas-containing feedcan also contain one or more C₄-hydrocarbons, especially methane,present either as unreacted reagent from natural gas used to produce thesyngas feed or added deliberately to the syngas feed. Thus, althoughmethane may be unreactive with the isosynthesis catalyst under thetypical conditions employed in the syngas conversion process, methanewill co-react with iso-C₄ hydrocarbons during aromatization toincorporate —CH_(x)— groups into the aromatic rings and reduce C₂+hydrocarbon cracking. In embodiments where methane is present or addedto the syngas-containing feed, the molar ratio of methane to CO in thefeed may be from 25 to 1, such as from 10 to 3.

Production of BTX from Syngas Via Isosynthesis/Aromatization

In certain aspects, the present disclosure provides a process forconverting syngas to a product mixture comprising benzene, toluene, andxylenes (BTX) by a combination of an isosynthesis reaction and anaromatization reaction. Para-xylene can be recovered from theBTX-containing product either directly or after post-treatment of theproduct by one of more of (a) xylene isomerization, (b) transalkylationwith at least one C₉+ aromatic hydrocarbon, and (c) para-selectivemethylation to increase the para-xylene concentration of the product.

Isosynthesis is a process for converting synthesis gas to a productcontaining C₄+ hydrocarbons, primarily branched aliphatic C₄₊hydrocarbons, at relatively high temperatures and pressures. The presentprocess may be conducted over a first catalyst with an isosynthesisfunctionality comprising at least one metal or compound thereof selectedfrom Ce, Zn, Zr, and Th. The isosynthesis catalyst may comprise a singleactive metallic species or may comprise a multimetallic, such as abimetallic or trimetallic, composition. For example, in one embodimentthe isosynthesis catalyst may comprise a combination of Ce and Zr,either in metallic or oxide form or a combination thereof. The amount ofactive metallic species present in the isosynthesis catalyst can varywidely depending on the particular metal or metals present in thecatalyst. For example, where the isosynthesis catalyst comprises acombination of Ce and Zr, the catalyst can contain from 0.1 to 10 Ce/Zrmol ratio; such as from 0.5 to 2 Ce/Zr mol ratio. In one embodiment, theisosynthesis catalyst comprises a mixed oxide of Ce and Zr having thegeneral formula Ce_(0.5)Zr_(0.5)O₂.

The active metallic species of the isosynthesis catalyst may beself-supporting or may be loaded on one or more supports selected from,for example, zinc oxide, magnesium oxide, alumina, silica, carbon, andmixtures thereof. Suitable carbon supports include activated carbon,carbon black, carbon nanotubes, and carbon nanofibers. In certainembodiments, the isosynthesis catalyst may further comprise one or moreelements selected from Groups 1 to 4 of the Periodic Table, such as Cs,K, and/or Ca, or compounds thereof, to act as stabilizers for the activemetallic species of the catalyst.

The present isosynthesis reaction comprises contacting asyngas-containing feed, as described above, optionally in the presenceof added methane, with a first catalyst with an isosynthesisfunctionality as described above under conditions including atemperature of least 350° C. and a pressure of least 1,500 kPa(absolute). Thus, in certain embodiments, the isosynthesis reaction isconducted at a temperature from 350° C. to 650° C. and a pressure from1,500 to 13,000 kPa (absolute), preferably a temperature from 450° C. to600° C. and a pressure from 2,000 to 7,000 kPa (absolute), and morepreferably a temperature from 500° C. to 550° C. and a pressure from2,500 to 5,000 kPa (absolute).

In the present process, the product of the isosynthesis reaction iscontacted with a second catalyst with an aromatization functionalitycomprising at least one molecular sieve and optionally at least onedehydrogenation component. The second catalyst comprises at least onemolecular sieve and, in certain aspects, at least one medium pore sizemolecular sieve having a Constraint Index of 2-12 (as defined in U.S.Pat. No. 4,016,218). Examples of such medium pore molecular sievesinclude ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35, ZSM-48, andmixtures and intermediates thereof. ZSM-5 is described in detail in U.S.Pat. No. 3,702,886 and Re. 29,948. ZSM-11 is described in detail in U.S.Pat. No. 3,709,979. A ZSM-5/ZSM-11 intermediate structure is describedin U.S. Pat. No. 4,229,424. ZSM-12 is described in U.S. Pat. No.3,832,449. ZSM-22 is described in U.S. Pat. No. 4,556,477. ZSM-23 isdescribed in U.S. Pat. No. 4,076,842. ZSM-35 is described in U.S. Pat.No. 4,016,245. ZSM-48 is more particularly described in U.S. Pat. No.4,234,231.

In other aspects, the second catalyst employed in the present processcomprises at least one molecular sieve of the MCM-22 family. As usedherein, the term “molecular sieve of the MCM-22 family” (or “material ofthe MCM-22 family” or “MCM-22 family material” or “MCM-22 familyzeolite”) includes one or more of:

molecular sieves made from a common first degree crystalline buildingblock unit cell, which unit cell has the MWW framework topology. (A unitcell is a spatial arrangement of atoms which if tiled inthree-dimensional space describes the crystal structure. Such crystalstructures are discussed in the “Atlas of Zeolite Framework Types”,Fifth edition, 2001, the entire content of which is incorporated asreference);

molecular sieves made from a common second degree building block, beinga 2-dimensional tiling of such MWW framework topology unit cells,forming a monolayer of one unit cell thickness, preferably one c-unitcell thickness;

molecular sieves made from common second degree building blocks, beinglayers of one or more than one unit cell thickness, wherein the layer ofmore than one unit cell thickness is made from stacking, packing, orbinding at least two monolayers of one unit cell thickness. The stackingof such second degree building blocks can be in a regular fashion, anirregular fashion, a random fashion, or any combination thereof; andmolecular sieves made by any regular or random 2-dimensional or3-dimensional combination of unit cells having the MWW frameworktopology.

Molecular sieves of the MCM-22 family include those molecular sieveshaving an X-ray diffraction pattern including d-spacing maxima at12.4±0.25, 6.9±0.15, 3.57±0.07 and 3.42±0.07 Angstrom. The X-raydiffraction data used to characterize the material are obtained bystandard techniques using the K-alpha doublet of copper as incidentradiation and a diffractometer equipped with a scintillation counter andassociated computer as the collection system.

Materials of the MCM-22 family include MCM-22 (described in U.S. Pat.No. 4,954,325), PSH-3 (described in U.S. Pat. No. 4,439,409), SSZ-25(described in U.S. Pat. No. 4,826,667), ERB-1 (described in EuropeanPatent No. 0293032), ITQ-1 (described in U.S. Pat. No. 6,077,498), ITQ-2(described in International Patent Publication No. WO97/17290), MCM-36(described in U.S. Pat. No. 5,250,277), MCM-49 (described in U.S. Pat.No. 5,236,575), MCM-56 (described in U.S. Pat. No. 5,362,697), andmixtures thereof. Related zeolite UZM-8 is also suitable for use as amolecular sieve component of the present catalyst.

In certain aspects, the molecular sieve employed in the aromatizationcatalyst may be an aluminosilicate or a substituted aluminosilicate inwhich part of all of the aluminum is replaced by a different trivalentmetal, such as gallium or indium.

In addition to the molecular sieve component, the aromatization catalystmay comprise at least one dehydrogenation component, e.g., at least onedehydrogenation metal. The dehydrogenation component is typicallypresent in an amount of at least 0.1 wt %, such as from 0.1 to 10 wt %,of the overall catalyst. The dehydrogenation component can comprise oneor more neutral metals selected from Groups 3 to 13 of the PeriodicTable of the Elements, such as Ga, In, Zn, Cu, Re, Mo, W, La, Fe, Ag,Pt, Pd, Ni, and/or one or more oxides, sulfides and/or carbides of thesemetals. The dehydrogenation component can be provided on the catalyst inany manner, for example by conventional methods such as impregnation orion exchange of the molecular sieve with a solution of a compound of therelevant metal, followed by conversion of the metal compound to thedesired form, namely neutral metal, oxide, sulfide, and/or carbide. Partor all of the dehydrogenation metal may also be present in thecrystalline framework of the molecular sieve.

In one embodiment, the aromatization catalyst comprises ZSM-5 containingfrom 0.1 to 10 wt % Zn.

In some embodiments, the aromatization catalyst is selectivated, eitherbefore introduction into the aromatization reactor or in-situ in thereactor, by contacting the catalyst with a selectivating agent. In oneembodiment, the catalyst is silica-selectivated by contacting thecatalyst with at least one organosilicon in a liquid carrier andsubsequently calcining the silicon-containing catalyst in anoxygen-containing atmosphere, e.g., air, at a temperature of 350° C. to550° C. A suitable silica-selectivation procedure is described in U.S.Pat. No. 5,476,823, the entire contents of which are incorporated hereinby reference. In another embodiment, the catalyst is selectivated bycontacting the catalyst with steam. Steaming of the zeolite is effectedat a temperature of at least about 950° C., preferably about 950° C. toabout 1075° C., and most preferably about 1000° C. to about 1050° C. forabout 10 minutes to about 10 hours, preferably from 30 minutes to 5hours. The selectivation procedure, which may be repeated multipletimes, alters the diffusion characteristics of the catalyst and mayincrease the xylene yield during the aromatization process.

In addition to, or in place of, silica or steam selectivation, thecatalyst may be subjected to coke selectivation. This optional cokeselectivation typically involves contacting the catalyst with athermally decomposable organic compound at an elevated temperature inexcess of the decomposition temperature of said compound but below thetemperature at which the crystallinity of the molecular sieve isadversely affected. Further details regarding coke selectivationtechniques are provided in the U.S. Pat. No. 4,117,026, incorporated byreference herein. In some embodiments, a combination of silicaselectivation and coke selectivation may be employed.

Aromatization of the isosynthesis reaction product may be conducted overa wide range of temperature and pressures, although generallytemperatures of least 350° C. are desirable. Thus, in certainembodiments, the aromatization reaction is conducted at a temperaturefrom 350° C. to 600° C. and a pressure from 1,500 to 10,000 kPa(absolute), for example at a temperature from 375° C. to 500° C. and apressure from 2000 to 7000 kPa (absolute). Desirably, the isosynthesisreaction and the aromatization reaction are conducted undersubstantially the same conditions.

The isosynthesis reaction and the aromatization reaction may beconducted in separate catalyst beds arranged in series or the samereaction vessel. Alternatively, the isosynthesis reaction and thearomatization reaction may be conducted in the same reaction bed withthe catalysts being stacked, mixed or combined into a singlemulti-functional catalyst particle. In such a case, it will beappreciated the iso-C₄₊ containing hydrocarbon product of theisosynthesis reaction may be instantaneously converted by thearomatization catalyst into a heavier aromatic-containing product.

Where the isosynthesis and aromatization reactions are conducted inseparate catalyst beds, the effluent from the isosynthesis reaction canbe subjected to one or more separation steps to remove unreactedcomponents, such as unreacted syngas, and reaction by-products, such aswater, CO₂ and H₂, before the iso-C₄₊ containing hydrocarbon product isforwarded to the aromatization reaction. Optionally, the unreactedsyngas is recycled to the isosynthesis reaction.

The effluent from the aromatization reaction comprises a C₅+ hydrocarbonproduct together with water, CO₂, H₂, and small quantities of C⁴⁻hydrocarbons. In certain embodiments, the C₅₊ hydrocarbon productcomprises from 1 to 50 wt % aromatics. The aromatization effluent can besubjected to one or more separation processes to remove unwantedby-products, such as water and CO₂, and to recover H₂, which can berecycled to the isosynthesis reaction, and C⁵⁻ hydrocarbons, which canbe used as fuel. At least part of the aromatic product in the effluentis then subjected to one or more post-treatments to increase theconcentration of para-xylene. Suitable post treatments include at leastone of (a) contacting with a xylene isomerization catalyst, (b)transalkylation with at least one C₉+ aromatic hydrocarbon, and (c)para-selective alkylation with methanol and/or carbon monoxide andhydrogen.

Where the at least one of post-treatment comprises contacting at leastpart of the aromatic product in the effluent with a xylene isomerizationcatalyst, the benzene and toluene components and any C₉₊ components inthe aromatization effluent may initially be removed, for example bydistillation, to produce a C₈-containing fraction. The C₈-containingfraction can then be fed to p-xylene separation section, where p-xyleneis recovered by adsorption or crystallization or a combination of bothand the residual p-xylene-depleted stream contacted with any knownxylene isomerization catalyst system operating in either the vapor phaseor liquid phase. A suitable xylene isomerization catalyst systemcomprises the liquid phase process described in U.S. Patent ApplicationPublication Nos. 2011/0263918 and 2011/0319688, the entire contents ofeach of which are incorporated herein by reference. Such a processemploys a molecular sieve xylene isomerization catalyst, such as ZSM-5or MCM-49, and operates at a temperature from 230° C. to 300° C., apressure from about 1300 to about 2100 kPa (absolute) and a weighthourly space velocity (WHSV) of from about 0.5 to about 10 hr⁻¹ toconvert the p-xylene-depleted stream back or close to equilibriumxylenes concentration with very low by-product formation. The isomerizedproduct can then be recycled to p-xylene separation section for furtherrecovery of p-xylene, while the benzene, toluene components and any C₉₊components removed from the aromatization effluent can be fed to thetransalkylation process described below.

Where the at least one of post-treatment comprises transalkylation of atleast part of the aromatic product in the effluent with at least one C₉+aromatic hydrocarbon, a C₈-containing fraction may initially be removedfrom the aromatization effluent, for example by distillation, andsupplied to the xylene recovery and isomerization process discussedabove. The remainder of the aromatization effluent, containing benzeneand toluene, is then supplied to a transalkylation process together withat least one C₉₊ aromatic hydrocarbon, generally from an externalsource. Any transalkylation process known to those skilled in the artcan be used, but one preferred process employs the multi-stage catalyticsystem described in U.S. Pat. No. 7,663,010, incorporated herein byreference in its entirety. Such a system comprises (i) a first catalystcomprising a first molecular sieve having a Constraint Index in therange of 3-12 and containing 0.01 to 5 wt % of at least one source of afirst metal element of Groups 6-10 of the Periodic Table and (ii). asecond catalyst comprising a second molecular sieve having a ConstraintIndex less than 3 and comprising 0 to 5 wt % of at least one source of asecond metal element of Groups 6-10 of the Periodic Table, wherein theweight ratio of the first catalyst for the second catalyst is in therange of 5:95 to 75:25 and wherein the first catalyst is locatedupstream of the second catalyst.

Examples of suitable molecular sieves having a Constraint Index of 3-12for use in the first catalyst include ZSM-5, ZSM-11, ZSM-22, ZSM-23,ZSM-35, ZSM-48, ZSM-57 and ZSM-58, with ZSM-5 being preferred. Examplesof suitable molecular sieves having a Constraint Index of less than 3for use in the second catalyst include zeolite beta, zeolite Y,Ultrastable Y (USY), Dealuminized Y (Deal Y), mordenite, ZSM-3, ZSM-4,ZSM-12, ZSM-18, NU-87 and ZSM-20, with ZSM-12 being preferred. Specificexamples of useful metals for each of the first and second catalystsinclude iron, ruthenium, osmium, nickel, cobalt, rhodium, iridium, andnoble metals such as platinum or palladium.

The transalkylation process can be conducted in any appropriate reactorincluding a radial flow, fixed bed, continuous down flow, or fluid bedreactor. The conditions in the first and second catalyst bed can be thesame or different but generally comprise a temperature from 100° C. to1000° C., preferably in the range of 300° C. to 500° C.; a pressure inthe range of 790 to 7000 kPa-a (kilo-Pascal absolute), preferably in therange of 2170 to 3000 kPa-a, a hydrogen to hydrocarbon molar ratio from0.01 to 20, preferably from 1 to 10; and a WHSV from 0.01 to 100 hr⁻¹,preferably in the range of 1-20 hr⁻¹.

Where the at least one of post-treatment comprises para-selectivealkylation of at least part of the aromatic product in the effluent withmethanol and/or carbon monoxide and hydrogen, a C₈₊-containing fractionmay initially be removed from the aromatization effluent, for example bydistillation, and supplied to the xylene recovery and isomerizationprocess discussed above. The remainder of the aromatization effluent,containing benzene and toluene, is then supplied to a para-selectivealkylation.

In certain aspects, methylation of the benzene and toluene in thearomatization effluent is conducted over an alkylation catalystcomprising a molecular sieve having a Diffusion Parameter for2,2-dimethylbutane of about 0.1-15 sec⁻¹, such as 0.5-10 sec⁻¹, whenmeasured at a temperature of 120° C. and a 2,2-dimethylbutane pressureof 60 torr (8 kPa). As used herein, the Diffusion Parameter of aparticular porous crystalline material is defined as D/r²×10⁶, wherein Dis the diffusion coefficient (cm²/sec) and r is the crystal radius (cm).The required diffusion parameters can be derived from sorptionmeasurements provided the assumption is made that the plane sheet modeldescribes the diffusion process. Thus for a given sorbate loading Q, thevalue Q/Q_(∞), where Q_(∞) is the equilibrium sorbate loading, ismathematically related to (Dt/r²)^(1/2) where t is the time (sec)required to reach the sorbate loading Q. Graphical solutions for theplane sheet model are given by J. Crank in “The Mathematics ofDiffusion”, Oxford University Press, Ely House, London, 1967.

The molecular sieve employed in the present alkylation process isnormally a medium-pore size aluminosilicate zeolite. Medium porezeolites are generally defined as those having a pore size of about 5 toabout 7 Angstroms, such that the zeolite freely sorbs molecules such asn-hexane, 3-methylpentane, benzene and p-xylene. Another commondefinition for medium pore zeolites involves the Constraint Index testwhich is described in U.S. Pat. No. 4,016,218, which is incorporatedherein by reference. In this case, medium pore zeolites have aConstraint Index of about 1-12, as measured on the zeolite alone withoutthe introduction of oxide modifiers and prior to any steaming to adjustthe diffusivity of the catalyst. Particular examples of suitable mediumpore zeolites include ZSM-5, ZSM-11, ZSM-12, ZSM-22, ZSM-23, ZSM-35,ZSM-48, and MCM-22, with ZSM-5 and ZSM-11 being particularly preferred.

The medium pore zeolites described above are particularly effective forthe present alkylation process since the size and shape of their poresfavor the production of p-xylene over the other xylene isomers. However,conventional forms of these zeolites have Diffusion Parameter values inexcess of the 0.1-15 sec⁻¹ range referred to above. However, therequired diffusivity for the catalyst can be achieved by severelysteaming the catalyst so as to effect a controlled reduction in themicropore volume of the catalyst to not less than 50%, and preferably50-90%, of that of the unsteamed catalyst. Reduction in micropore volumeis derived by measuring the n-hexane adsorption capacity of thecatalyst, before and after steaming, at 90° C. and 75 torr n-hexanepressure.

Steaming of the zeolite is effected at a temperature of at least about950° C., preferably about 950 to about 1075° C., and most preferablyabout 1000 to about 1050° C. for about 10 minutes to about 10 hours,preferably from 30 minutes to 5 hours.

To effect the desired controlled reduction in diffusivity and microporevolume, it may be desirable to combine the zeolite, prior to steaming,with at least one oxide modifier, such as at least one oxide selectedfrom elements of Groups 2 to 4 and 13 to 16 of the Periodic Table.Preferably, said at least one oxide modifier is selected from oxides ofboron, magnesium, calcium, lanthanum, and more preferably, phosphorus.In some cases, the zeolite may be combined with more than one oxidemodifier, for example a combination of phosphorus with calcium and/ormagnesium, since in this way it may be possible to reduce the steamingseverity needed to achieve a target diffusivity value. In someembodiments, the total amount of oxide modifier present in the catalyst,as measured on an elemental basis, may be between about 0.05 and about20 wt %, and preferably is between about 0.1 and about 10 wt %, based onthe weight of the final catalyst.

Where the modifier includes phosphorus, incorporation of modifier intothe catalyst is conveniently achieved by the methods described in U.S.Pat. Nos. 4,356,338; 5,110,776; 5,231,064; and 5,348,643, the entiredisclosures of which are incorporated herein by reference. Treatmentwith phosphorus-containing compounds can readily be accomplished bycontacting the zeolite, either alone or in combination with a binder ormatrix material, with a solution of an appropriate phosphorus compound,followed by drying and calcining to convert the phosphorus to its oxideform. Contact with the phosphorus-containing compound is generallyconducted at a temperature of about 25° C. and about 125° C. for a timebetween about 15 minutes and about 20 hours. The concentration of thephosphorus in the contact mixture may be between about 0.01 and about 30wt %. Suitable phosphorus compounds include, but are not limited to,phosphonic, phosphinous, phosphorus and phosphoric acids, salts andesters of such acids, and phosphorous halides.

After contacting with the phosphorus-containing compound, the porouscrystalline material may be dried and calcined to convert the phosphorusto an oxide form. Calcination can be carried out in an inert atmosphereor in the presence of oxygen, for example, in air at a temperature ofabout 150° C. to 750° C., preferably about 300° C. to 500° C., for atleast 1 hour, preferably 3-5 hours. Similar techniques known in the artcan be used to incorporate other modifying oxides into the catalystemployed in the alkylation process.

In addition to the zeolite and modifying oxide, the catalyst employed inthe alkylation process may include one or more binder or matrixmaterials resistant to the temperatures and other conditions employed inthe process. Such materials include active and inactive materials suchas clays, silica and/or metal oxides such as alumina. The latter may beeither naturally occurring or in the form of gelatinous precipitates orgels including mixtures of silica and metal oxides. Use of a materialwhich is active, tends to change the conversion and/or selectivity ofthe catalyst and hence is generally not preferred. Inactive materialssuitably serve as diluents to control the amount of conversion in agiven process so that products can be obtained economically and orderlywithout employing other means for controlling the rate of reaction.These materials may be incorporated into naturally occurring clays,e.g., bentonite and kaolin, to improve the crush strength of thecatalyst under commercial operating conditions. Said materials, i.e.,clays, oxides, etc., function as binders for the catalyst. It isdesirable to provide a catalyst having good crush strength because incommercial use it is desirable to prevent the catalyst from breakingdown into powder-like materials. These clay and/or oxide binders havebeen employed normally only for the purpose of improving the crushstrength of the catalyst.

Naturally occurring clays which can be composited with the porouscrystalline material include the montmorillonite and kaolin family,which families include the subbentonites, and the kaolins commonly knownas Dixie, McNamee, Georgia and Florida clays or others in which the mainmineral constituent is halloysite, kaolinite, dickite, nacrite, oranauxite. Such clays can be used in the raw state as originally mined orinitially subjected to calcination, acid treatment or chemicalmodification.

In addition to the foregoing materials, the porous crystalline materialcan be composited with a porous matrix material such as silica-alumina,silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia,silica-titania as well as ternary compositions suchassilica-alumina-thoria, silica-alumina-zirconia,silica-alumina-magnesia, and silica-magnesia-zirconia.

The relative proportions of porous crystalline material and inorganicoxide matrix vary widely, with the content of the former ranging fromabout 1 to about 90% by weight and more usually, particularly when thecomposite is prepared in the form of beads, in the range of about 2 toabout 80 wt % of the composite. Preferably, the matrix materialcomprises silica or a kaolin clay.

The alkylation catalyst used in the present process may optionally beprecoked. The precoking step is may be carried out by initially loadinguncoked catalyst into the methylation reactor. Then, as the reactionproceeds, coke is deposited on the catalyst surface and thereafter maybe controlled within a desired range, typically from about 1 to about 20wt % and preferably from about 1 to about 5 wt %, by periodicregeneration by exposure to an oxygen-containing atmosphere at anelevated temperature.

In certain aspects, methylation of the benzene and toluene in thearomatization effluent is effected with a methylating agent comprisingmethanol and/or a mixture of carbon monoxide and hydrogen. In the lattercase, the molar ratio of carbon monoxide to hydrogen in the methylatingagent may comprise from 1 to 4, such as from 1.5 to 3.0. Suitableconditions for the methylation reaction a temperature from 350° C. to700° C., preferably from 500° C. to 600° C., a pressure of from 100 and7000 kPa absolute, a weight hourly space velocity of from 0.5 to 1000hr⁻¹, and a molar ratio of toluene to methanol (in the reactor charge)of at least about 0.2, e.g., from about 0.2 to about 20. The process maysuitably be carried out in fixed, moving, or fluid catalyst beds. If itis desired to continuously control the extent of coke loading, moving orfluid bed configurations are preferred. With moving or fluid bedconfigurations, the extent of coke loading can be controlled by varyingthe severity and/or the frequency of continuous oxidative regenerationin the catalyst regenerator.

Using the present process, toluene can be alkylated with methanol so asto produce para-xylene at a selectivity of at least about 80 wt % (basedon total C₈ aromatic product) at a per-pass aromatic conversion of atleast about 15 wt % and a trimethylbenzene production level less than 1wt %. Unreacted benzene and toluene and methylating agent and a portionof the water by-product may be recycled to the methylation reactor andheavy byproducts routed to fuels dispositions. The C8 fraction is routedto a para-xylene recovery unit, which typically operates by fractionalcrystallization or by selective adsorption (e.g. Parex or Eluxyl) torecover a para-xylene product stream from the alkylation effluent andleave a para-xylene-depleted stream containing mainly C₇ and C₈hydrocarbons. The para-xylene-depleted stream may be isomerized andrecycled to the para-xylene recovery unit.

The description above support one or more of the following more specificEmbodiments.

Embodiment 1

A process for producing xylenes, the process comprising: (a) providing afeed comprising hydrogen and carbon monoxide, in which the molar ratioof hydrogen to carbon monoxide is from 0.5 to 6; (b) contacting the feedwith (i) a first catalyst comprising at least one metal or compoundcontaining a metal selected from the group consisting of Ce, Zn, Zr, andTh, and (ii) a second catalyst, which may be the same as or differentthan the first catalyst, comprising at least one medium pore sizemolecular sieve under conditions including a temperature of at least350° C. and a pressure of at least 1500 kPa (absolute) effective toproduce a reaction effluent containing benzene, toluene, and xylenes;and (c) subjecting at least part of the reaction effluent to at leastone of (i) contacting with a xylene isomerization catalyst, (ii)transalkylation with at least one C₉+ aromatic hydrocarbon, and (iii)alkylation with methanol and/or carbon monoxide and hydrogen underconditions to produce p-xylene.

Embodiment 2

The process of Embodiment 1, wherein the feed further comprises methane.

Embodiment 3

The process of Embodiment 1 or Embodiment 2, wherein the first catalystcomprises Ce_(0.5)Zr_(0.5)O₂.

Embodiment 4

The process of any one of Embodiments 1 to 3, wherein the secondcatalyst comprises at least one molecular sieve having a ConstraintIndex of 1-12.

Embodiment 5

The process of any one of Embodiments 1 to 4, wherein the at least onemolecular sieve of the second catalyst comprises ZSM-5.

Embodiment 6

The process of any one of Embodiments 1 to 5, wherein the secondcatalyst comprises at least one metal or compound thereof, wherein themetal is selected from the group consisting of Ga, In, Zn, Cu, Re, Mo,W, La, Fe, Ag, Pt, Pd, and Ni.

Embodiment 7

The process of any one of Embodiments 1 to 6, wherein the secondcatalyst is silica-selectivated.

Embodiment 8

The process of any one of Embodiments 1 to 7, wherein the secondcatalyst is steam-selectivated.

Embodiment 9

The process of any one of Embodiments 1 to 8, wherein the first andsecond catalysts are located in different reaction beds.

Embodiment 10

The process of any one of Embodiments 1 to 9, wherein the first andsecond catalysts are different but are located in the same reaction bed.

Embodiment 11

The process of any one of Embodiments 1 to 9, wherein the first andsecond catalysts are combined into a single multi-functional catalyst.

Embodiment 12

The process of any one of Embodiments 1 to 11, wherein the conditions in(b) comprise a temperature from 350 to 600° C. and a pressure from 1,500to 10,000 kPa (absolute).

Embodiment 13

The process of any one of Embodiments 1 to 12, wherein (c) comprisescontacting at least part of the xylenes in the reaction effluent withxylene isomerization catalyst.

Embodiment 14

The process of any one of Embodiments 1 to 12, wherein (c) comprisestransalkylating at least part of the benzene and/or toluene in thereaction effluent with at least one C₉+ aromatic hydrocarbon.

Embodiment 15

The process of any one of Embodiments 1 to 12, wherein (c) comprisesalkylating at least part of the benzene and/or toluene in the reactioneffluent with methanol and/or hydrogen and carbon monoxide in thepresence of a third catalyst comprising at least one molecular sievehaving a Diffusion Parameter for 2,2-dimethylbutane of from 0.1 to 15sec⁻¹ when measured at a temperature of 120° C. and a 2,2-dimethylbutanepressure of 60 torr (8 kPa).

Embodiment 16

A catalyst system for the production of para-xylene comprising: (a) afirst catalyst comprising at least one metal or compound containing ametal selected from the group consisting of Ce, Zn, Zr, and Th, and (b)a second catalyst, which may be the same as or different than the firstcatalyst, comprising at least one medium pore size molecular sieve andat least one metal selected from Groups 10-14 of the Periodic Table orcompound thereof, wherein the first and second catalysts are locatedwithin the same reactor bed.

Embodiment 17

The catalyst system of Embodiment 16 wherein the first and secondcatalysts are different and are physically mixed in the same reactorbed.

Embodiment 18

The catalyst system of Embodiment 16 wherein the first and secondcatalysts are combined into a single multi-functional catalyst.

Embodiment 19

The catalyst system of any one of Embodiments 16-18, wherein the secondcatalyst comprises at least one metal or compound thereof, wherein themetal is selected from the group consisting of Ga, In, Zn, Cu, Re, Mo,W, La, Fe, Ag, Pt, Pd, and Ni.

Embodiment 20

The catalyst system of any one of Embodiments 16-19, wherein the metalof the second catalyst is present in an amount of about 0.1 to 10 wt %.

Embodiment 21

The catalyst system of any one of Embodiments 16-20, wherein the secondcatalyst comprises silica-selectivated ZSM-5.

All patents, test procedures, and other documents cited herein,including priority documents, are fully incorporated by reference to theextent such disclosure is not inconsistent and for all jurisdictions inwhich such incorporation is permitted.

While the illustrative forms disclosed herein have been described withparticularity, it will be understood that various other modificationswill be apparent to and can be readily made by those skilled in the artwithout departing from the spirit and scope of the disclosure.Accordingly, it is not intended that the scope of the claims appendedhereto be limited to the examples and descriptions set forth herein butrather that the claims be construed as encompassing all the features ofpatentable novelty which reside herein, including all features whichwould be treated as equivalents thereof by those skilled in the art towhich this disclosure pertains.

When numerical lower limits and numerical upper limits are listedherein, ranges from any lower limit to any upper limit are contemplated,and are expressly within the scope of the invention. The term“comprising” is synonymous with the term “including”. Likewise whenevera composition, an element or a group of components is preceded with thetransitional phrase “comprising”, it is understood that we alsocontemplate the same composition or group of components withtransitional phrases “consisting essentially of,” “consisting of”,“selected from the group of consisting of,” or “is” preceding therecitation of the composition, component, or components, and vice versa.

The invention claimed is:
 1. A process for producing xylenes, theprocess comprising: (a) providing a feed comprising methane, hydrogen,and carbon monoxide, in which the molar ratio of hydrogen to carbonmonoxide is from 0.5 to 6 and in which the molar ration of methane tocarbon monoxide is from 25 to 1; (b) contacting the feed with (i) afirst catalyst comprising at least one metal or compound containing ametal selected from the group consisting of Ce, Zn, Zr, and Th, and (ii)a second catalyst comprising at least one medium pore size molecularsieve, wherein the contacting in (b)(i) and (b)(ii) is carried out underconditions including a temperature of at least 350° C. and a pressure ofat least 1500 kPa (absolute) effective to produce a reaction effluentcontaining benzene, toluene, and xylenes; and (c) subjecting at leastpart of the reaction effluent to at least one of (i) contacting with axylene isomerization catalyst, (ii) transalkylation with at least oneC₉+ aromatic hydrocarbon, and (iii) alkylation with methanol and/or thecombination of carbon monoxide and hydrogen under conditions to producep-xylene.
 2. The process of claim 1, wherein the first catalystcomprises Ce_(0.5)Zr_(0.5)O₂.
 3. The process of claim 1, wherein thesecond catalyst comprises at least one molecular sieve having aConstraint Index of 1-12.
 4. The process of claim 1, wherein the atleast one medium pore size molecular sieve of the second catalystcomprises ZSM-5.
 5. The process of claim 1, wherein the second catalystcomprises at least one metal or compound thereof, wherein the metal isselected from the group consisting of Ga, In, Zn, Cu, Re, Mo, W, La, Fe,Ag, Pt, Pd, and Ni.
 6. The process of claim 1, wherein the secondcatalyst is silica-selectivated.
 7. The process of claim 1, wherein thesecond catalyst is steam-selectivated.
 8. The process of claim 1,wherein the first and second catalysts are located in different reactionbeds.
 9. The process of claim 1, wherein the first and second catalystsare different but are located in the same reaction bed.
 10. The processof claim 1, wherein the first and second catalysts are combined into asingle multi-functional catalyst.
 11. The process of claim 1, whereinthe conditions in (b) comprise a temperature from 350 to 600° C. and apressure from 1,500 to 10,000 kPa (absolute).
 12. The process of claim1, wherein (c) comprises contacting at least part of the xylenes in thereaction effluent with xylene isomerization catalyst.
 13. The process ofclaim 1, wherein (c) comprises transalkylating at least part of thebenzene and/or toluene in the reaction effluent with at least one C₉+aromatic hydrocarbon.
 14. The process of claim 1, wherein (c) comprisesalkylating at least part of the benzene and/or toluene in the reactioneffluent with methanol and/or hydrogen and carbon monoxide in thepresence of a third catalyst comprising at least one molecular sievehaving a Diffusion Parameter for 2,2-dimethylbutane of from 0.1 to 15sec⁻¹ when measured at a temperature of 120° C. and a 2,2-dimethylbutanepressure of 60 torr (8 kPa).